In 12 years of harassment by a succession of teen-agers, nobody had ever come to his door and threatened to kill him, Roman G. Welzant testified today, until that awful snowy night in January.

Haltingly, sometimes in tears, the 68-year-old Welzant described the night and the fear that propelled him, gun in hand, into a confrontation in which he killed a teen-ager and seriously wounded another.

Taking the stand for the first time in a murder trial in which he has been cast as a symbol of the persecuted elderly, Welzant said it was a shouted threat, punctuated by the shattering of glass in his front door, that finally drew him out onto the cold suburban street.

"I opened the door," he said. "I didn't know who he was. He put a foot up, smashed the door. He said, You f---- old man, I'll kill you!'"

"I was afraid if I didn't go out and find who he was and have him arrested, he'd come back and kill me."

Welzant's testimony and that of his 64-year-old wife Genevieve, provided the most dramatic moments so far in the four days of his trial in circuit court here.

Welzant's actions the night of Jan. 4 in the Baltimore suburb of Eastwood have drawn national attention.

From all over the country, more than 200 contributions have come for his defense -- mostly in small bills, but including one anonymous cashier's check for $1,000. Usually they are accompanied by letters of support from the donors, often elderly, offering prayers and sympathy for Welzant's ordeal.

The prosecution case rests largely on the testimony of several teen-agers who participated in the night's events and on other evidence that purports to show Welzant's intentions. "Come on up here. I'll shoot you," one youth testified Welzant said shortly before the shootings occurred.

In one phone call to police, Welzant said, according to the transcript, "Get a car out here, will you, I'll take a gun out to these people." A police officer who came to the Welzant house on Overview Avenue said Welzant told him "he'd have to take a gun himself to get anything done." Today Welzant disputed the officer's recollection of his remark.

Their voices at times barely audible, the elderly husband and his wife told how their house seemed to shake under the repeated barrages of snowballs and obscenities hurled their way and of their frustration with police who never seemed to arrive on time or care enough when they did arrive.

Survivors of the Great Depression with limited education but a firm belief in the American dream, they moved from East Baltimore to the modest row house development in 1953. They lived there peacefully for years, as their three children grew to adulthood and left home.

Genevieve Welzant recalled the troubles that beset them, one after another, from then on: Beer bottles and cans "all over the front and back every day"; a hedge set on fire; a freshly painted car scraped by a nail from one end to the other; all but three windows in the house broken at one time or another; her flower boxes of 15 years taken into the field behind their house and crushed.

The Welzants could never identify the vandals and the teen-agers knew him only as "Camera man," a distant figure who, they said, energed from his house to yell and snap their pictures. The nickname, he said yesterday, was only a product of "their guilty consciences."

To the Welzants, the inexplicable harassment seemed to reach a crescendo the night of Jan. 4, when shortly after 9 p.m., he testified "the house was violently struck with snowballs. I was afraid they'd break the windows and leave us with the house cold."

He called for them to stop and, four times that night -- twice before the shootings -- he called the police. He fired a warning shot from his porch, and later from the street. They went away once, to get more beer, it turned out. The police came and left. Welzant would have to identify the youths, he was told, as he had been told many times before. The youths returned and the barrage resumed.

His wife testified she jumped up and started screaming, "They're breaking in." She called her son, who called police. "I guess God was with me because I pushed the right numbers," she said.

Welzant said he sent her upstairs for her own protection. "She was shaking and her face was in spasms," Welzant testified, his head in his hands.He told her to lock herself in the bedroom but she "started heaving" in the bathroom instead, she said.

Welzant, meanwhile, had left the house, armed and determined to identify the youth who moments before, he said, had kicked in his door and threatened to kill him.

The confrontation ended, according to prosecution witnesses, when Welzant, untouched, killed 18-year-old Albert Kahl Jr., and wounded James Willey, 16.

Welzant told it differently. The youth who kicked in the door, he said, "kept saying, 'Come on old man, I'll fight you, I'll beat the hell out of you.'"

And suddenly he said, another youth, Willey, "slammed me downward. I hit the sidewalk. The gun fired. He let go. The next instant, Albert Kahl came pouncing on me. He sent me face down toward the ground. He had hold of me some way. The gun went off again. When Kahl let me go, he didn't say anything. He stood straight up."

Welzant testified under cross examination, "I was afraid they were gonna smash my head open and then stomp on me."

Welzant retreated to his house and made two more calls to police. In the first, at 10:36 p.m., he told them to hurry because "they're breaking my house. . . . They're trying to get me now."

Then he realized he had shot at least one of the youths and, at 10:38 p.m., he made his final call.

"Have you got a police car for 424 Overview yet?" Welzant said.

"They're on their way out there, sir. The roads are very slick," a voice answered.

"I think you'll need an ambulance too," Welzant said. "I think one of them got shot. I shot one of them."

During cross examination, prosecutors tried to get Welzant to repudiate parts of the statement he gave to police immediately after the shooting. Welzant stood by the "essence" of his statement.

The trial resumes Monday.